I have been dealing with heat soak issues on my TR4. I have done some research on the rally style wing vents such as sold by Revington TR. They are pricey and it means a lot of body work to install. So I came up with an alternative.

I'm using Land Rover side vents. They are a reasonable facsimile to the factory vents and with careful cutting can be installed without having repaint the bodywork.
I'll fabricate the tunnel pieces and the backing flange using carbon fiber (styrofoam and vacuum bag).

I picked up a Triumph Herald grille off ebay which I'll use to make the insert.

Wonder what the difference in cooling effect would be...side vents versus hood louvres.
There is also the question of water ingress, depending if you drive in rain.
Easier to buy another hood than repair fenders and inner guards if the side vent cooling is not to your expectation.

Interesting your plug didn't crush under vacuum. I considered this method for my air box but in the end used machinable wax as a plug
to make the mold. I love working with carbon fiber. I made velocity stacks for my old Mikunis that were so light it just seemed unreal.
The part is looking good though.
jg

Stef: I actually tried something similar, but, I realized air in was not the problem. Air out is. With the honda drive train there is not much room around the engine for air to go out. So, I pushing air in and it does not flow out. I tested this by placing a short 2 x 4 between the bonnet and latch to hold the hood open about 2 inches, temps went right down. With air scoop open cockpit temperatures went right up.

JimG: Really nice work with the carbon fiber. Mine is not nearly as nice, but, is hidden.

Oliver: Summers in Texas never look quite like that

Things are coming along:

The Herald grille I sourced is rough (too rough to polish out), so I'm painting the grille inserts the metallic silver color of the bonnet stripe, the wing vent surrounds are painted the color of the car.

I go to the scary part cutting through the fender and inner fender without incident.

I made a Top Hat section out of carbon fiber to slide into the tube I made that attached to the outer vent section. Then I cut. I

After cutting and grinding (very carefully) it was time to assemble the whole mess. The Land Rover vents have clips to hold them to the body, but, I used a little Auto Body Panel adhesive to hold everything in place. I painted the Triumph Herald grille bits the color of the bonnet stripe (dark grey metallic silver) only with the vertical slats in flat black.